1. Field of the Invention
The present invention relates to a method for forming a specular protective film.
2. Related Background Art
It is common practice to use a rotary polygon mirror for polarizing a light beam in a laser scanning optical system for use with a laser printer or a laser copying machine, as described in Japanese Patent Publication No. 62-36219.
Such a rotary polygon mirror is generally made of aluminum alloy, plastics, or glass, with its reflecting surface (specular surface) having a transparent intense reflecting film (protective film) applied thereon.
When the reflecting surface is an aluminum specular surface, an anodic oxide film has been applied as a protective film. In this case, it can function well because of having a good adherence to a substratum of aluminum alloy.
When an anodic oxide film is formed in such a transparent single layer film to serve as a protective layer of the specular surface such that the intensity of reflected light may be maximum, the optical film thickness is m.cndot./2cos.theta. (.lambda.: wavelength of incident light, .theta.: angle of incidence, m: positive integer) and the angle of incidence is at a center of scan range, as described in Japanese Laid-Open Patent Application No. 58-184903.
However, in the conventional art as above described, when a specular part is formed of aluminum alloy, the refractive index n.sub.0 as a mirror can be represented as EQU n.sub.0 =n.sub.2 -i.cndot.k
(k: extinction coefficient, i=.sqroot.-1)
where n.sub.2 is a refractive index of the aluminum alloy. Here, n.sub.0 is a complex refractive index, but when the angle of incidence is at a center of the scan range for light incident on the specular protective film, the intensity of reflected light is set to be maximum, without regard to the complex refractive index n.sub.0. Thus, the conventional art has a problem that there is a large difference between intensities of reflected lights from the central portion of the scan range and its peripheral portion, when the incident angle changes, so that it can not be used practically.